(a) Field of the Invention
The present invention relates to an automatic transmission for a vehicle, and more particularly, to an automatic transmission that obtains shift-stages of at least nine forward speed and at least one reverse speed using a minimal number of parts.
(b) Description of the Related Art
In the field of an automatic transmission, more multiplicity of shift-stages is useful for enhancement of fuel consumption and drivability of a vehicle. Recently, increase of oil price is triggering a competition in enhancing fuel consumption of a vehicle. Accordingly, research has been conducted in regards to an engine to achieve weight reduction and enhance fuel consumption by downsizing and research regarding an automatic transmission has been conducted to simultaneously provide better drivability and fuel consumption by achieving more shift stages. To achieve more shift-stages for an automatic transmission, the number of parts is typically increased, which may deteriorate installability, production cost, weight and/or power flow efficiency. Therefore, to maximally enhance fuel consumption of an automatic transmission having more shift-stages, it is important that improved efficiency is derived by less number of parts.
In this background, an eight-speed automatic transmission has been introduced recently and a planetary gear train for an automatic transmission enabling more shift-stages is being researched. An automatic transmission of eight or more shift-stages typically includes three to four planetary gear sets and five to six control elements (frictional elements), and may become lengthy, thereby deteriorating installability. Accordingly, disposing planetary gear sets in parallel or employing dog clutches instead of wet-type control elements has been attempted. However, such an arrangement may not be widely applicable, and using dog clutches may deteriorate shift-feel.
The above information disclosed in this section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.